Bluetooth: Pass all message parameters to mgmt_start_discovery

The mgmt_start_discovery command contains the type of discovery that
should be started so this should be passed to the start_discovery
function. This patch doesn't yet add any action depending on the type of
the requested discovery.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>